This invention relates to an encoder for modems using QAM and PSK type modulation, and more particularly to an encoder without multipliers.
Modems using QAM and PSK type modulation have become standard in the data communications field. Briefly, such modems transmit and receive multi-valued modulated symbols which are distinguishable by phase and radial amplitude. Such symbols may be represented as points on a complex eye diagram. In the past the symbols have been generated by respectively feeding the X- and Y- coordinates of the points representing the symbols into two transversal digital filters. The outputs of the filters were used to modulate, respectively, a sine and cosine wave of the desired frequency. The two waves were then combined to produce the desired symbol. Such systems may be inaccurate because the transversal digital filters contain a plurality of multipliers which can degrade the transmitted symbols. Furthermore, the system is difficult and costly to implement with digital circuits.